1. Field of the Invention
The present invention relates to a display device comprising an element (hereinafter referred to as a light-emitting element) in which a light-emitting material is interposed between a pair of electrode, and to a method of manufacturing thereof. More particularly, the present invention relates to a sealing structure of a display device using a light-emitting material that generates electroluminescence (EL) and a method of manufacturing the same.
2. Description of the Related Art
In recent years, development of a display device (EL display device) using a light-emitting element (hereinafter referred to as an EL element) which utilizes electroluminescent phenomenon of a light-emitting material has been advanced. Since the light-emitting element of the EL display device is a self-luminous type, a backlight, which is used for a conventional liquid crystal display device, is unnecessary. In addition, the EL display device has merits of the wide viewing angle, the excellent visibility and the like.
It has been said that the EL element emits photons as follows. By applying a voltage to an organic compound layer interposed between a pair of electrodes, electrons injected from a cathode and positive holes injected from an anode recombine together at a center of luminescence in the organic compound layer to form excited molecules, and hence, energy is released to emit photons when the excited molecules return to the base state. There are known two different excited states: a singlet excited state; and a triplet excited state. Luminescence can be generated through either of the states.
Although there are inorganic light-emitting materials and organic light-emitting materials as the material used for the EL element, the organic light-emitting materials which can be driven at lower voltage than the inorganic light-emitting materials has been attracting much attention.
However, when the EL element comprising an organic material is driven for a certain period, luminance and light emitting properties such as nonuniformity of light emission are drastically deteriorated as compared with those in the initial state, which results in a problem of low reliability. The low reliability of the EL element is a limiting factor in practical application of the EL material.
Further, moisture and oxygen which penetrate from outside into the EL element are another factor of deteriorating the reliability of the EL element.
In an EL display device (panel) using an EL element, moisture penetrating into the interior of the device causes a serious degraded reliability, which further reads to a dark spot, shrinkage, and deterioration in luminance from a periphery of The EL display device. The dark spot is a phenomenon in which luminance is partly degraded (which includes nonluminous portion), and is generated when a hole is formed in an upper electrode. Meanwhile, the shrinkage is a phenomenon in which luminance is deteriorated from edges of a pixel.
Accordingly, a display device comprising a structure for preventing the above-mentioned deteriorations of the EL element has been researched and developed. In order to prevent the foregoing problems, there is a method in which the EL element is accommodated in an airtight container and a desiccant is provided in an airtight space (for example, see patent document 1).
[Patent Document 1]
Japanese Patent Application Laid-Open No. Hei 9-148066
Further, there is another method in which a sealing material is formed on an insulator with the EL element formed thereon, and an airtight space surrounded by a covering material and the sealing material is filled with a filler such as a resin so as to shield the EL element from an exterior portion (for example, see patent document 2).
[Patent Document 2]
Japanese Patent Application Laid-Open No. 13-203076
FIG. 5 shows a top view of the EL display device as disclosed in the patent document 2. Reference numeral 401 surrounded by a doted line denotes a source side driving circuit, reference numeral 402 denotes a gate side driving circuit, reference numeral 403 denotes a pixel portion, and reference numeral 409 denotes a flexible printed circuit (FPC). Further, reference numeral 404 denotes a covering material, reference numeral 405 denotes a first sealing material, and reference numeral 406 denotes a second sealing material. FIG. 6 shows a cross sectional view of a conventional EL display device as illustrated in FIG. 5 (the second sealing material 406 is not illustrated therein). In FIG. 6, reference numeral 800 is a substrate, reference numeral 801 is an electrode, reference numeral 811 is a pixel electrode, reference numeral 812 is an insulating film, reference numeral 813 is an EL layer, reference numeral 814 is a cathode, and reference numeral 815 is an EL element. As illustrated in FIG. 6, in a sealing region, the EL element is encapsulated in an interior portion with the sealing material 817.
According to the patent documents 1 and 2, in the sealing region, the EL element is protected from moisture exists outside by providing the sealing material as depicted in FIG. 6.
With respect to the patent document 1, however, when the structure in which the EL element is accommodated in the airtight container is taken, the EL display device is grown in size for the size of the airtight container. Although the EL display device is only grown in size, the light-emitting area is not changed. Therefore, the advantage of a thin EL display device, which dispenses with a backlight, cannot be utilized.
Furthermore, with respect to the patent document 2, since the sealing material is applied to a substrate to form the airtight space in the sealing region, it is inevitable that the EL display device is grown in size.
As set forth above, as the area of the sealing region is increased, nonluminous regions are further increased. Accordingly, it is inevitable that the display device is grown in size in order to obtain a light-emitting portion with a desired area.
In view of the above mentioned problems, a display device with a narrow frame in which the sealing region is made as narrow as possible has been researched and developed (for example, see patent document 3).
[Patent Document 3]
Japanese Patent Application Laid-Open No. 2002-329576
In the patent document 3, a sealing pattern used for sealing is formed over a substrate with a depression formed thereon. When the width of the sealing pattern in the sealing region is narrowed in order to achieve a narrower frame portion, since the surface area through which the sealing pattern and the substrate contact becomes large, reduction in the bond strength therebetween can be suppressed.
However, in the patent document 3, the sealing material is applied over the substrate in the sealing region as well as the patent documents 1 and 2, and therefore there is a limitation on a narrower frame formation.
Further, there is another method in which the sealing material is directly applied to a film such as an interlayer film, and a protective film as substitute for the substrate so as to eliminate the sealing region for applying the sealing material. An EL display device manufactured according to this method is illustrated in FIG. 7. An enlarged cross sectional view of an edge of the sealing region taken along a line C-C′ in FIG. 5 is illustrated in FIG. 14.
As depicted in FIG. 14, a first coating film 53, a second coating film 54, a third coating film 55, and a fourth coating film 56 are laminated over a substrate 50 in a display device. Further, a sealing material 52 is applied thereon. This structure allows to reduce the sealing region. The first coating film 53, the second coating film 54, the third coating film 55, and the fourth coating film 56 corresponds to a base film, a gate insulating film, a protective film, an interlayer film, a conductive film, and the like, respectively.
However, when the sealing material for sealing is formed over a laminated film as illustrated in FIG. 14, each laminated film is directly in contact with an ambient air outside of the display device. Therefore, moisture and oxygen existing outside of the display device penetrate into the display device through the each laminated film. In addition, when a material having high moisture permeability such as acrylic is used as the interlayer film, more moisture and oxygen penetrate into the display device.
Moisture and oxygen penetrate through the acrylic included in the interlayer film or top and bottom interfaces of acrylic by using the acrylic as a path. The moisture and oxygen ultimately reach to the EL element via a disconnection portion in a contact hole due to poor film formation properties of source and drain electrodes and the like. The interior of the EL display device and the EL element are contaminated by these moisture and oxygen, thereby causing various deteriorations such as deterioration of the electric characteristics, a dark spot, and shrinkage.